Connection method for mems navigation unit for computer-assisted surgery

ABSTRACT

A computer-assisted surgery (CAS) navigation assembly comprises a micro-electromechanical sensor (MEMS) navigation unit having one or more MEMS to provide at least orientation data. A support receives the MEMS navigation unit therein, the support being adapted to be mounted on the instrument in a fixed orientation relative to established navigated features of the instrument. At least two mating ball-in-socket features are disposed between the MEMS navigation unit and the support at opposed ends thereof for releasably engaging the MEMS navigation unit in precise orientational alignment within the receptacle, the at least two mating ball-in-socket features comprising catches aligned along an axis extending between the opposed ends, at least one of the catches being a biased catch. A method of connecting a MEMS navigation unit with a mating support fixed to a CAS instrument navigated by the CAS system is also provided.

CROSS-REFERENCE TO RELATED APPLICATION

The present patent application claims priority on U.S. ProvisionalApplication No. 61/579,873, incorporated herewith by reference.

TECHNICAL FIELD

The present application relates generally to computer-assistedorthopedic surgery systems using micro-electromechanical sensors.

BACKGROUND

The use of micro-electromechanical sensors (MEMS), rather than moretraditional optically-tracked sensors for example, for the purposes ofcommunicating with a computer-assisted surgery (CAS) system such as tonavigate tools, bones, reference markers, etc. within the surgical fieldis becoming more desirable because such MEMS sensors are not limited byline-of-sight requirements of previously used optical sensors. TheseMEMS, which may for example include accelerometers and/or gyroscopes,are able to wirelessly communicate with the CAS system with which theyare employed, or are equipped with a processor and user interfaceproviding guidance to a user, i.e., the MEMS unit is part of a portableCAS system that is directly on the CAS instrument. Accordingly, the CASsystem is able to determine at least orientation information of the MEMSunit, and therefore able to locate and track (i.e. navigate) the tool orbone to which the MEMS unit is fastened.

One of the steps required to navigate any tracked bone reference orsurgical tool using a CAS system, including one which employs MEMS, isto “calibrate” the CAS instrument (ex: either a bone reference or tool)by precisely locating the position and/or orientation of the sensorrelative that of the CAS instrument to which it is fastened. In theevent that the sensor is detached from the CAS instrument, it istypically necessary to re-calibrate the assembly once the sensor isagain fixed in its normal position relative to the instrument.

It may be desirable to be able to switch MEMS from one CAS instrument toanother, however this becomes problematic because each time the MEMS isremoved and then re-fastened, either onto another instrument or evenback onto the same original instrument from which it was detached, a newcalibration step must be performed in order to ensure that the exactrelative position of the MEMS and the CAS instrument to which it isfastened are determined by the CAS system.

Accordingly, there is a need for a MEMS-navigated CAS instrument whichenables the MEMS to be readily removed and re-attached to the instrumentwith repeatable precision and accuracy such that the relative alignmentand orientation of the instrument and the MEMS will remain constant,thereby avoiding the need to re-calibrate the entire instrument in theevent that the MEMS sensor is removed and/or re-attached.

SUMMARY

It is therefore an object of the present invention to provide animproved MEMS-navigated CAS instrument and an associated method ofconnecting such a MEMS navigation unit to such a CAS instrument.

Therefore, in accordance with one aspect of the present disclosure,there is provided a computer-assisted surgery (CAS) navigation assemblyadapted for navigating an instrument, the navigation assemblycomprising: a micro-electromechanical sensor (MEMS) navigation unithaving one or more MEMS to provide at least orientation data; a supportreceiving the MEMS navigation unit therein, the support being adapted tobe mounted on the instrument in a fixed orientation relative toestablished navigated features of the instrument; and at least twomating ball-in-socket features disposed between the MEMS navigation unitand the support at opposed ends thereof for releasably engaging the MEMSnavigation unit in precise orientational alignment within thereceptacle, the at least two mating ball-in-socket features comprisingcatches aligned along an axis extending between the opposed ends, atleast one of the catches being a biased catch.

Further in accordance with the first aspect, the support comprises areceptacle for receiving a portion of the MEMS navigation unit therein,the ball-in-socket features being located in the receptacle.

Still further in accordance with the first aspect, the receptaclecomprises at least one planar surface, with the MEMS navigation unitbeing in coplanar abutment with the at least one planar surface whenreleasably engaged in the fixed orientation.

Still further in accordance with the first aspect, the receptaclecomprises at least two planar surfaces for coplanar with correspondingsurfaces of the MEMS navigation unit, the two planar surfaces beingtransverse with respect to one another.

Still further in accordance with the first aspect, the receptaclecomprises three planar surfaces arranged in a U, with normals of theplanar abutment surfaces being transverse relative to said axis.

Still further in accordance with the first aspect, one of the catches isa fixed catch.

Still further in accordance with the first aspect, the fixed catch has ageometry defined by a sphere quarter merging with a half-cylinder.

Still further in accordance with the first aspect, the fixed catchprotrudes from the MEMS navigation unit.

Still further in accordance with the first aspect, the biased catch ispart of the support, and wherein a socket cooperating with the biasedcatch is on the MEMS navigation unit.

Still further in accordance with the first aspect, the socket is defineda raised plateau on the MEMS navigation unit.

Still further in accordance with the first aspect, the MEMS navigationunit comprises a ramp portion adjacent to the socket and transitioningto the raised plateau for guiding the biased catch into the socketduring assembly.

Still further in accordance with the first aspect, an elastomersurrounds a periphery of the raised plateau in the releasableengagement.

Still further in accordance with the first aspect, a latch feature is onthe support for latching engagement of the MEMS navigation unit to thesupport.

Still further in accordance with the first aspect, the latch featurecomprises a lever for disengagement of the latch feature from the MEMSnavigation unit.

In accordance with a second aspect of the present disclosure, there isprovided a method of connecting a micro-electromechanical sensor (MEMS)navigation unit of a computer-assisted surgery (CAS) system with amating support fixed to a CAS instrument navigated by the CAS system,the method comprising: releasably engaging the MEMS navigation unitwithin the support in precise relative orientational alignment,comprising: aligning and matingly engaging a pair of ball-in-socketfeatures disposed between the MEMS navigation unit and the support atopposed ends thereof, the pair of ball-in-socket features being alignedrelative to each; and snap-fitting a biased catch of at least one ofsaid ball-in-socket features within a corresponding socket to constrainand align the relative orientation of the MEMS navigation unit and thesupport together about a first axis.

Further in accordance with the second aspect, releasably engaging theMEMS navigation unit further comprises abutting at least one planarsurface on the MEMS navigation unit against a planar surface on areceptacle of the support, the planar surfaces lying in a substantiallycommon plane to constrain the MEMS navigation unit and the support aboutat least a second axis.

Still further in accordance with the second aspect, the MEMS navigationunit is latched to the support when releasably engaging the MEMSnavigation unit to the support.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a MEMS navigation unit and its matingreceptacle, in accordance with one aspect of the present disclosure;

FIG. 2 a is a side elevation view of the MEMS navigation unit andreceptacle of FIG. 1 engaged together;

FIG. 2 b is a cross-sectional view taken through line A-A in FIG. 2 a;

FIG. 3 a is a first perspective view of the MEMS navigation unit andreceptacle of FIG. 1, showing in detail an upper end connectiontherebetween;

FIG. 3 b is a first perspective view of the MEMS navigation unit andreceptacle of FIG. 1, showing in detail a lower end connectiontherebetween;

FIG. 4 a is a second perspective view of the MEMS navigation unit andreceptacle of FIG. 3 a, showing a socket feature for connection of theupper end of the MEMS navigation unit to the receptacle;

FIG. 4 b is a second perspective view of the receptacle of FIG. 3 b,showing a socket feature for connection with the lower end of the MEMSnavigation unit;

FIG. 5 is a cross-sectional view of the socket connection between theMEMS navigation unit and the mating receptacle;

FIG. 6 a is front elevation view of the mated MEMS navigation unit andthe receptacle;

FIG. 6 b is a cross-sectional view taken through line B-B in FIG. 6 a;

FIG. 6 c is a detailed cross-sectional view taken from region C in FIG.6 b;

FIG. 6 d is a detailed cross-sectional view taken from region D in FIG.6 b;

FIG. 7 a is a side elevational view of the MEMS navigation unit beingconnected with the mating receptacle;

FIG. 7 b is a side elevational view of the MEMS navigation unit beinglocated in position within the mating receptacle;

FIG. 8 is a cross-sectional view showing a first embodiment of asecuring feature between the mated MEMS navigation unit and thereceptacle;

FIG. 9 a is a cross-sectional view showing a second embodiment of thesecuring feature between the mated MEMS navigation unit and thereceptacle; and

FIG. 9 b is a detailed cross-sectional view taken from region F in FIG.9 a.

DETAILED DESCRIPTION

Referring to FIG. 1, computer-assisted surgery (CAS) navigation assemblyof the present disclosure includes generally an electronicmicro-electromechanical sensor (MEMS) navigation unit 12 and acorresponding connecting support 14 which is adapted to be fixed inplace on a CAS instrument, such as a tool or bone reference for example,which is to be navigated using the CAS system. In an embodiment, theconnecting receptacle 14 is part of the CAS instrument, wherebyreference to the receptacle 14 may include the CAS instrument. The MEMSnavigation unit 12 and the receptacle 14 are engaged together in themanner described hereinbelow in further detail. The MEMS navigation unit12 includes one or more of an accelerometer and/or a gyroscope, and isoperable to communicate (e.g., wirelessly or not) with the CAS systemwith which the navigation assembly 10 is employed. According to anotherembodiment, the MEMS navigation unit 12 has a processor and visualinterface capable of producing navigation output, i.e., withoutnecessarily communicating with a CAS system in that the MEMS navigationunit 12 is the CAS system. Accordingly, the CAS system is able todetermine at least orientation information of the MEMS unit, andtherefore able to locate and track (i.e. navigate) the CAS instrument towhich the MEMS unit 12 is fastened via its connecting dock or receptacle14. In yet another embodiment, the MEMS navigation unit 12 is of thetype having a preset calibration setting, in that its orientation aboutat least one axis may be known when activated and/or reset.

The support 14 is shown as being a connecting dock or receptacle 14, andis accordingly adapted to be fixed in position and orientation to theCAS instrument to be navigated using the CAS system. For clarity,reference will be made to the support 14 as the receptacle 14, inaccordance with the illustrated embodiment. Accordingly, the receptacle14 is disposed in a fixed orientation relative to the establishednavigated feature of the instrument to which it is fastened. Asdescribed below, the MEMS navigation unit 12 is readily engaged withinthis receptacle 14 in a manner which provides a quick-connect andquick-disconnect type interconnection, however even given this ease ofconnection between the two components they are removably engagedtogether in a fashion which enables their relative alignment andorientation to remain constant and exact each and every time. As such,the MEMS navigation unit 12 can be disconnected from the receptacle 14and then re-attached thereto, without needing to re-calibrate the entireCAS instrument. This permits, for example, the same MEMS navigation unit12 to be used during a surgery for navigating several different CASinstruments, each having the same receptacle 14 mounted thereon. Asingle MEMS navigation unit 12 could therefore be used, if necessary ordesirable, to navigate several different instruments within the surgicalfield, provided of course they do not need to be tracked simultaneously.

To enable this, the MEMS navigation unit 12 is oriented in an exactposition relative to the receptacle 14 using a predetermined and definedsystem of orthogonal planes, as seen in FIGS. 2 a-2 b, which are formedby the shape and configuration of the receptacle 14. This permits aprecise and repeatable relation between the MEMS navigation unit 12 andits receptacle 14 to be established each and every time the MEMSnavigation unit 12 is engaged in place within the receptacle 14, withoutrequiring a new calibration procedure to be performed.

This precision alignment between the MEMS navigation unit 12 and thereceptacle 14 of the CAS instrument is made possible by at least twoconnection devices 16 and 30, each of which includes cooperatingfeatures on each of the receptacle 14 and MEMS navigation unit 12, suchas to interconnect the two components of the present navigation assembly10 in repeatably precise orientation relative to each other about allthree orthogonal axes (X, Y and Z).

As seen in FIGS. 2 a-5, the first connection device 16 includes a pairof ball-in-socket features 18 and 19 which are vertically alignedrelative to each other along the X-axis, as seen in FIG. 2 b. The matingball-in-socket features 18, 19 between each end of the MEMS navigationunit 12 and the receptacle, forming the first connection device 16, arethus used to constrain and align the relative orientation of the twocomponents about the z-axis.

Referring to the mating ball-in-socket features 18 and 19 in moredetail, as seen in FIGS. 3 a-5, the first ball-in-socket feature 18comprises a biased catch or catch ball element 20 and the secondball-in-socket feature 19 comprises a fixed catch or ball element 26,each of which engage respective sockets 24, 28 as will be seen.

In the depicted embodiment, best seen in FIG. 2 b for example, thebiased ball-in-socket feature 18 having the biased catch element 20 islocated at a top end of the MEMS unit 12 and the receptacle 14; howeverit may alternately be disposed at the opposite, bottom, end of thecomponents.

As best seen in FIG. 5, the first, or biased, the biased catch element20 of the ball-in-socket feature 18 may include, in at least thisembodiment, a displaceable ball 20 which is biased by a spring 22 orother suitable biasing element (such as an elastomer, etc.). The spring22 accordingly permits the sprung ball 20 to be forced inwardly awayfrom its normally outwardly biased position (as shown in FIG. 5), inorder to be able to interconnect the two components 12 and 14, butreturns the ball 20 back into its outwardly biased engaged position. Thespring or biasing element 22 is compliant within an established forcerange such as to permit compression thereof, and thus connection betweenthe ball 20 and its mating socket 24, without the use of high forcewhile nevertheless maintaining the biased contact between the MEMS unit12 and the receptacle 14.

When the two components 12 and 14 are being interconnected, therefore,once the ball 20 disposed on the receptacle 14 is aligned with itsmating socket 24 formed in the MEMS unit 12, the ball 20 accordinglysprings back outwardly such as to matingly engage with the socket 24 andthereby interconnect the two components 12 and 14 at this connectionpoint 16. Although in the present embodiment the sprung ball 20 of thebiased ball-in-socket feature 18 is disposed on the receptacle and thecorrespondingly shaped mating socket 24 is disposed in the MEMS unit 12,it is to be understood that the opposite configuration may also be used.

As best seen in FIG. 4 a, the socket 24 which receives the sprung ball20 of the biased ball-in-socket feature 18 may comprise be formed in aplateau raised from a remainder of the connection device 16. Anopen-topped slot 23 is defined in the raised plateau and has an open end25 and a closed end 27, however circular or other shaped receptacles 24may also be used. A ramp 27 a may be formed between the raised plateauand the remainder of the connection device 16. The ramp 27 a is adjacentto closed end 27. The biased ball, or catch element, 20 of the matingball-in-socket feature 18 accordingly enables a “snap”-fit connectionbetween the MEMS navigation unit 12 and the receptacle 14 at therespective end at which this feature 18 is disposed. It is pointed outthat, despite a full sphere being illustrated being shown in theillustrated embodiment, other geometries may be used, such as afrusto-sphere, etc. The expression “ball” covers segments of a ball, ofa sphere including at least a portion of the outer surface of the ball,sphere, etc. The expression “socket” covers a concavity shaped to havesome mating contact with the ball.

As noted above, and seen in FIGS. 3 b and 4 b, the opposedball-in-socket feature 19 of the first connection device 16, which isdisposed at the opposite end of the two components 12 and 14 from thebiased ball-in-socket feature 18, includes a protruding fixed (i.e.rigid or non-biased) catch element 26 that is integrally formed with thebase of the MEMS unit 12. The rigid catch element 26 is received withinits corresponding mating socket 28 formed in the receptacle 14. However,the opposite configuration may also be used (i.e. the protruding fixedcatch element 26 may be located on the receptacle 14 and the matingsocket 28 may be formed in the body of the MEMS unit 12). As observed inFIG. 3 b, the rigid catch element 26 has a ball portion and morespecifically a quarter of a sphere or ball, with a half-cylinder base,although a ball, a frusto-sphere or half-ball, etc could be used, all ofwhich fall within the scope and definition of ball-and-socket feature.The combination of the ball and half-cylinder base may provideadditional contact surface between components of the ball-in-socketfeature 19.

Regardless, as shown in FIGS. 7 a-7 b, the rigid ball-in-socketconnection feature 19 is the first of the two alignment features 18, 19to be engaged when connected the MEMS unit 12 into the receptacle 14(step “A”), following which the biased ball-in-socket feature 18 isengaged by pivoting the MEMS navigation unit 12 inward into thereceptacle 14 (step “B”) until the point where the biased alignmentfeature 18 snaps into engagement, thereby securing the two components12, 14 together in mated engagement as shown in FIG. 7 b.

In another possible embodiment, however, the first connection device 16may comprise two biased ball-in-socket features 18, rather than havingone fixed feature 19 and one biased feature 18 as per the depictedembodiment described above. Regardless, the first connection device 16includes a pair of mating ball-in-socket features which releasablyinterconnect the MEMS navigation unit 12 and the receptacle 14 and whichare vertically aligned relative to each other along the X-axis, such asto constrain and align the relative orientation of the two components12, 14 about the z-axis (see FIG. 2 a).

As noted above, the MEMS navigation unit 12 and the receptacle 14 areinterconnected in a repeatably precise orientation relative to eachother by at least two connection devices 16 and 30, each of whichincludes shared and cooperating features on each of the receptacle 14and MEMS navigation unit 12.

Referring now to FIGS. 6 a-6 d, the second of these connection devices30 constrains the relative orientation of the MEMS navigation unit 12and the receptacle 14 about the X-axis and the Y-axis. This secondconnection device 30 comprises the abutting contact between the rearplanar surface 32 of the MEMS navigation unit 12 and the front planarsurface 34 within the receptacle 14. These two planar surfaces 32, 34accordingly abut and lay flat one on top of the other, albeit in avertical orientation, when the MEMS navigation unit 12 and thereceptacle 14 are engaged together as shown in FIG. 6 b. By abutting thetwo planar surfaces 32 and 34 such that they lie in a substantiallycommon plane XY as shown in FIG. 6 b, in as much as two solid planarsurfaces can, the relative orientation of the MEMS unit 12 and thereceptacle 14 is thereby constrained about the X and Y axes. The U-shapeof the receptacle 14 is well suited for retaining the MEMS unit 12.

In order to make sure that the respective planar surfaces 32 and 34 ofthe MEMS unit 12 and the receptacle 14 remain in precise abuttedcontact, additional angular retention features 36 and 38 between theball-in-socket connections 18 and 19 are additionally provided. As seenin FIGS. 6 b-6 d, the first angular retention feature 36 is formed onthe MEMS unit 12 opposed from the biased connection 18 at the upper endthereof, and more particularly is defined at the closed end 27 of theslot 23 which forms the socket 24 receiving the biased ball 20 (see FIG.4 a). Acting opposed to this first angular retention feature 36 (i.e.facing in a opposite direction), is a second angular retention feature38 which is jointly formed at the connection point between thereceptacle 14 and the MEMS unit 12 at the lower end thereof, where thefixed catch element 26 is matingly received within the correspondingsocket 28. This second angular retention feature 38 may be provided, forexample, by the curved end wall 39 of the slot or socket 28 in the lowerend of the receptacle 14. The angular retention features 36 and 38therefore act in concert such as to keep the respective planar surfaces32 and 34 of the MEMS unit 12 and the receptacle 14 in forced abuttedcontact, which in turn ensures that the orientations of the twocomponents 12 and 14 are constrained about the X and Y axes. It isobserved that the connection of the MEMS unit 12 to the receptacle 14 ispredictable, using the connection features described above. Hence, it ispossible to program the MEMS unit 12 of the preset type with anorientation related to the predicted orientation of the MEMS unit 12 inthe receptacle 14. In doing so, the MEMS unit 12 could be pre-calibratedin such a way its orientation about at least one axis is known relativeto a navigated feature of the instrument, when the preset MEMS unit 12is activated or reset.

As described above, and referring again to FIGS. 7 a-7 b, the method ofconnected the MEMS navigation unit 12 and the receptacle 14 is performedby first aligning the rigid ball-in-socket connection feature 19 at thelower end of the two components 12 and 14 and matingly engaging thisconnection feature 19 by sliding the MEMS unit in direction A, and thenpivoting or rotating the upper end of the MEMS unit 12 into thereceptacle 14 in direction B such as to matingly engage the biasedconnection feature 18 at the upper end of the two components. The biasedcatch/ball 20 will be helped into reaching the socket 24 by moving alongthe ramp 27 a. Once the biased catch/ball 20 of the upper connectionfeature 18 is snapped into engagement with its mating socket 24, therespective planar surfaces 32 and 34 of the MEMS unit 12 and thereceptacle 14 are then in forced abutted contact, and the two components12 and 14 are fastened together as shown in FIG. 7 b. The MEMSnavigation unit 12 and the receptacle 14 are thereby connected inprecise engagement such that they are constrained in fixed position andorientation relative to each other.

While the interconnection system as described so far for fastening theMEMS navigation unit 12 within the receptacle 14 may be sufficient tomaintain their alignment with precision and accuracy, in at least oneembodiment of the present disclosure an additional, secondary,connection system is also provided to help maintain the MEMS navigationunit 12 within its receptacle 14 within established alignment precisionand accuracy, which may be particularly useful during impaction andoscillatory vibration which sometimes occurs during surgery.

Accordingly, referring particularly to FIGS. 8 to 9 b, and also seen inFIG. 6 b, a secondary connection system 40 is provided between the MEMSnavigation unit 12 and the receptacle 14 which includes a compliantmember 42 which is disposed between the upper surface 41 of the MEMSunit 12 and the facing upper rim 44 of the receptacle 14, such as toprovide additional constraint in at least in a direction substantiallyparallel to the X-axis (see FIGS. 2 a-2 b). The compliant member 42 mayinclude, for example, a wire-form or O-ring for example, and may eitherbe substantially rigid or may be at least somewhat elasticallydeflectable. In either case, however, the compliant member 42 providesadditional constraint in at least the X-axis direction to limit relativemovement between the MEMS unit 12 and the receptacle 14, and may alsoprovide damping therebetween such as to help absorb any vibrations.

In the event of impact or oscillatory vibration of the CAS tool to whichthe receptacle 14 is fixed, should the MEMS navigation unit 12 becomeslightly displaced within, or even momentarily disengaged from, thereceptacle 14 (for example such that the biased catch/ball 20 becomesomewhat disengaged form its mating socket 24), MEMS navigation unit 12will contact the compliant member 42 which will limit such displacementwithin the receptacle 14. The permitted displacement before thecompliant member 42 comes into contact with the MEMS unit 12 maycorrespond to the limit at which the biased ball 20 can automaticallyreturn into its socket 24. During insertion of the MEMS unit 12 into thereceptacle 14, the compliant member 42 will be moved upwards within itscorresponding slot 46 in the upper rim 44 of the receptacle 14, as theMEMS navigation unit 12 is slid into its engaged position within thereceptacle 14, whereupon the compliant member 42 will return to itsoriginal position as shown in FIG. 8.

In an alternate embodiment, an alternate secondary connection system 50may be provided in lieu of, or possibly even in addition to, theconnection system 40 described above. The alternate secondary connectionsystem 50 also includes a compliant member in the form of aspring-return lever 52 which is mounted to the upper rim 44 of thereceptacle 14 and includes a blade-spring 54 which is used to ensurethat the lever 52 remains in continuous engaged contact with the topsurface of the MEMS unit 12. The spring-return lever 52 operates in muchthe same manner as the compliant member 42 described above, in that itensures that the MEMS unit 12 remains engaged in position andorientation within the receptacle 14 even if the other connectionmechanisms (such as the biased catch/ball 20 of the connection device18) become slightly disengaged. Accordingly, similar to the wire-form orother element of the compliant member 42, the spring-return lever 12 ismoved upwards as the MEMS unit 12 is pushed into place within thereceptacle, and then returns into a latched position (see FIG. 9 a)wherein it is engaged with the top side edge of the MEMS navigation unit12 once the MEMS unit 12 is completely connected in place within thereceptacle 14. As seen in FIG. 9 b, the lever 52 includes a latchfeature 58 at its outer edge which engages the correspondingly shapedridge 56 on the top side edge of the MEMS unit 12. The matching geometryof the ridge 56 on the MEMS unit 12 and the latch feature 58 on thelever 52 ensures that the MEMS unit 12 will not be disconnected from thereceptacle during impaction and/or oscillatory vibration. To be able todisconnect the MEMS unit 12 from the receptacle 14, the lever 52 must bemanually and voluntarily lifted by the user. This accordingly providesan additional secure and fail-safe interconnection between the MEMSnavigation unit 12 and the receptacle 14.

The terms “top” and “bottom”, “upward” and “downward”, “vertical”, etc.,are generally used herein with reference to the orientation of theassembly 10 as shown in the drawings for ease of description purposesonly, however it is to be understood that depending on the orientationof the instrument to which the receptacle 14 is fixed, these directionsmay not actually correspond to a true or absolute vertical, upwardand/or downward direction.

The above description is meant to be exemplary only, and one skilled inthe art will recognize that changes may be made to the embodimentsdescribed without departing from the scope of the invention disclosed.Still other modifications which fall within the scope of the presentinvention will be apparent to those skilled in the art, in light of areview of this disclosure, and such modifications are intended to fallwithin the appended claims.

1. A computer-assisted surgery (CAS) navigation assembly adapted fornavigating an instrument, the navigation assembly comprising: amicro-electromechanical sensor (MEMS) navigation unit having one or moreMEMS to provide at least orientation data; a support receiving the MEMSnavigation unit therein, the support being adapted to be mounted on theinstrument in a fixed orientation relative to established navigatedfeatures of the instrument; and at least two mating ball-in-socketfeatures disposed between the MEMS navigation unit and the support atopposed ends thereof for releasably engaging the MEMS navigation unit inprecise orientational alignment within the receptacle, the at least twomating ball-in-socket features comprising catches aligned along an axisextending between the opposed ends, at least one of the catches being abiased catch.
 2. The computer-assisted surgery (CAS) navigation assemblyaccording to claim 1, wherein the support comprises a receptacle forreceiving a portion of the MEMS navigation unit therein, theball-in-socket features being located in the receptacle.
 3. Thecomputer-assisted surgery (CAS) navigation assembly according to claim2, wherein the receptacle comprises at least one planar surface, withthe MEMS navigation unit being in coplanar abutment with the at leastone planar surface when releasably engaged in the fixed orientation. 4.The computer-assisted surgery (CAS) navigation assembly according toclaim 3, wherein the receptacle comprises at least two planar surfacesfor coplanar with corresponding surfaces of the MEMS navigation unit,the two planar surfaces being transverse with respect to one another. 5.The computer-assisted surgery (CAS) navigation assembly according toclaim 4, wherein the receptacle comprises three planar surfaces arrangedin a U, with normals of the planar abutment surfaces being transverserelative to said axis.
 6. The computer-assisted surgery (CAS) navigationassembly according to claim 1, wherein one of the catches is a fixedcatch.
 7. The computer-assisted surgery (CAS) navigation assemblyaccording to claim 6, wherein the fixed catch has a geometry defined bya sphere quarter merging with a half-cylinder.
 8. The computer-assistedsurgery (CAS) navigation assembly according to claim 6, wherein thefixed catch protrudes from the MEMS navigation unit.
 9. Thecomputer-assisted surgery (CAS) navigation assembly according to claim1, wherein the biased catch is part of the support, and wherein a socketcooperating with the biased catch is on the MEMS navigation unit. 10.The computer-assisted surgery (CAS) navigation assembly according toclaim 9, wherein the socket is defined a raised plateau on the MEMSnavigation unit.
 11. The computer-assisted surgery (CAS) navigationassembly according to claim 10, wherein the MEMS navigation unitcomprises a ramp portion adjacent to the socket and transitioning to theraised plateau for guiding the biased catch into the socket duringassembly.
 12. The computer-assisted surgery (CAS) navigation assemblyaccording to claim 10, further comprising an elastomer surrounding aperiphery of the raised plateau in the releasable engagement.
 13. Thecomputer-assisted surgery (CAS) navigation assembly according to claim1, further comprising a latch feature on the support for latchingengagement of the MEMS navigation unit to the support.
 14. Thecomputer-assisted surgery (CAS) navigation assembly according to claim13, wherein the latch feature comprises a lever for disengagement of thelatch feature from the MEMS navigation unit.
 15. The computer-assistedsurgery (CAS) navigation assembly according to claim 1, furthercomprising the instrument.
 16. A method of connecting amicro-electromechanical sensor (MEMS) navigation unit of acomputer-assisted surgery (CAS) system with a mating support fixed to aCAS instrument navigated by the CAS system, the method comprising:releasably engaging the MEMS navigation unit within the support inprecise relative orientational alignment, comprising: aligning andmatingly engaging a pair of ball-in-socket features disposed between theMEMS navigation unit and the support at opposed ends thereof, the pairof ball-in-socket features being aligned relative to each; andsnap-fitting a biased catch of at least one of said ball-in-socketfeatures within a corresponding socket to constrain and align therelative orientation of the MEMS navigation unit and the supporttogether about a first axis.
 17. The method according to claim 16,wherein releasably engaging the MEMS navigation unit further comprisesabutting at least one planar surface on the MEMS navigation unit againsta planar surface on a receptacle of the support, the planar surfaceslying in a substantially common plane to constrain the MEMS navigationunit and the support about at least a second axis.
 18. The methodaccording to claim 16, further comprising latching the MEMS navigationunit to the support when releasably engaging the MEMS navigation unit tothe support.